I've never heard anyone say 'fabby.' In the past, 'fab!' was quite popular with young people. It's not used much these days. It's out of fashion. Don't use it. You'll sound as if you are from the 1960s ! (But you can use 'fabulous!" of course)

We don't. lol I suppose if someone were trying to talk like a hippy. "That coat's pretty fab, man." Tip. Don't use fab or fabby with an English native; at least not an American. Maybe those Brits might prefer the word. lol

I've never heard fabby in Australia, but possibly fab was used in the 90's as slang. I can't say it's a popular expression though.